Experimentelle Untersuchung von Kohärenz und Verschränkung in Josephson-Phasenqubits

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It has been experimentally demonstrated that a variety of micro- and nano-scale superconducting electronic circuits containing Josephson junctions display quantum-coherent dynamics at low temperatures. These circuits are now viewed as very promising candidates to be used as qubits in a future solid state quantum computer. The simplest type of superconduction qubits is the so-called phase qubit, which makes use of the quantum states of a current biased Josephson tunnel junction. These quantum states can be coupled by resonant microwaves to form a controllable two-level systems. We would like to experimentally study individual and coupled Josephson phase qubits. Very recent experiments have shown that these quantum devices can be prepared, controlled and read out by microwave and dc pulses in a manner similar to nuclear spins. Our ultimate goal is to demonstrate coherent manipulation of individual Josephson qubits and create an entanglement between two coupled qubits. In order to achieve this goal, we will study the coupling of Josephson phase qubits to the environment by using tunable interaction between the qubit and on-chip microwave resonators. The relaxation time and the dephasing time of the qubit in the presence of the microwave-tailored environment will be determind. Two Josephsons phase qubits will be coupled capacitively in order to create an entanglement, which will be controlled by dc and microwave pulses. We intend also to study the interaction of the Josephson qubit with the quantized electromagnetic field modes of a high-Q on-chip resonator. We will experimentally characterize the coupling between these two quantum systems and investigate possible mechanisms to create and control their entanglement.